1. Field of the Invention
The present invention relates to a temperature sensing circuit using a thermopile sensor and, more particularly, to a temperature sensing circuit using a thermopile sensor, which is able to detect temperature accurately with regard to an offset of an amplifier included therein.
2. Discussion of Related Art
In general, a thermopile sensor is a device which generates a voltage corresponding to the difference between a temperature of infrared rays incident from an object and a surrounding temperature of the sensor. This thermopile sensor is usually employed as a device for sensing the temperature of a food placed in the cooking room of a mircrowave oven. However, a signal detected by the thermopile sensor is too weak to be directly used for controlling cooking in the microwave oven. Accordingly, the signal is needed to be amplified to an appropriate level.
A conventional temperature sensing circuit is explained below with reference to the attached drawing. FIG. 1 shows the configuration of a conventional temperature sensing circuit. Referring to FIG. 1, the conventional temperature sensing circuit is composed of a temperature detecting circuit 70 including a thermopile sensor 11 which generates a voltage value (Vb) corresponding to the difference between a temperature of infrared rays incident from an object and a surrounding temperature of the sensor, and a temperature amplifier 12 for compensating for an increase or decrease in voltage Vd with relation to the surrounding temperature within the output voltage of thermopile sensor 11.
Temperature amplifier 12 is composed of an amplifier 12a for amplifying the output of thermopile sensor 11, a temperature compensator 12b for amplifying output voltage Vb of thermopile sensor 11 and generating compensated voltage Vd, an adder 12c for adding an output of amplifier 12a to the voltage compensated by temperature compensator 12b, and an amplifier 12d for amplifying the output of adder 12c to generate a temperature sensing voltage Vo.
The operation of the conventional temperature sensing circuit is described below. Thermopile sensor 11 generates a voltage Va corresponding to the temperature of infrared rays incident from the object. Output voltage Va of thermopile sensor 11 varies with its surrounding temperature. The surrounding temperature is compensated using a temperature compensating device such as diode or thermistor which is arranged on one side of thermopile 11 or around it. The output value of the temperature compensating device corresponds to a resistance value when the temperature compensating device is a thermistor, and it corresponds to a voltage value when the device is a diode.
FIG. 1 shows a case where a diode is used as the temperature compensating device. Thermopile sensor 11 generates voltage Va corresponding to the temperature of infrared rays incident from the object. Amplifier 12a amplifies the output of thermopile sensor 11, and temperature compensator 12b amplifies output voltage Vb of the temperature compensating device (not shown). Adders 12c adds output voltage Vc of amplifier 12a to output voltage Vd of temperature compensator 12b. Here, the output of adder 12c becomes a voltage corresponding to the temperature of the object, which is obtained by removing the effect of the surrounding temperature of thermopile from output voltage Va of thermopile sensor 11. Amplifier 12d connected to the output terminal of adder 12c amplifies the output voltage of adder 12c and sends resulting value Vo to a microcomputer 80. Microcomputer 80 detects the temperature of the object according to output voltage Vo of temperature amplifier 12.
The aforementioned conventional temperature sensing circuit has the following problems. First of all, it requires the amplifier for amplifying the output of the thermopile sensor and the plurality of amplifiers for compensating for the variation in the surrounding temperature, resulting in complicated circuit configuration. Furthermore, the temperature compensating device and amplifiers are added, increasing the cost. Moreover, the output voltage of the thermopile sensor is amplified together with the offset values of the amplifiers so that it cannot be correctly calculated, deteriorating accuracy in temperature detection.